1. Field of the Invention
The present invention relates to a portable radio terminal such as a portable telephone set or a portable information terminal which has a foldable body, and more particularly to a technique for optimization of an antenna characteristic of a foldable portable radio terminal.
2. Description of the Related Art
Various foldable portable radio terminals are conventionally known, and as one of such foldable portable radio terminals, such a foldable portable telephone set as shown in FIGS. 7 and 8 is conventionally known.
Referring to FIGS. 7 and 8, the conventional foldable portable telephone set 20 includes an upper body 21a and a lower body 21b connected for opening and closing pivotal movement by a hinge mechanism 22, and a telescopic antenna 23 provided on the upper body 21a. 
In the foldable portable telephone set 20, the upper body 21a and the lower body 21b can be pivotally folded relative to each other around the hinge mechanism 22. FIG. 7 shows the foldable portable telephone set 20 in an unfolded state while FIG. 8 shows the foldable portable telephone set 20 in a folded state.
Referring to FIG. 7, a display section 25 including a receiver 24 and an LCD is provided on the upper body 21a, and a transmitter 26 and a keyboard 27 which is used to input a telephone number and other data are provided on the lower body 21b. 
FIG. 9 shows an internal structure of the foldable portable telephone set 20.
Referring to FIG. 9, the foldable portable telephone set 20 includes, in addition to the antenna 23, receiver 24, display section 25, transmitter 26 and keyboard 27, a radio section 28 for performing demodulation and other processes for a radio signal received by the antenna 23, a folded state detection circuit 29 for detecting whether or not the foldable portable telephone set is in a folded state and generating a detection signal corresponding to a result of the detection, an LCD driver 30 for driving the display section 25, an antenna matching circuit 32 for determining an antenna characteristic of the antenna 23, and a control section 33 for controlling the receiver 24, transmitter 26, radio section 28 and LCD driver 30.
The foldable portable telephone set 20 operates in the following manner.
A radio signal received by the antenna 23 is subject to demodulation and other processes by the radio section 28 and is issued as a voice signal from the receiver 24 under the control of the control section 33.
On the other hand, upon transmission, voice of a user is sent through the transmitter 26 to the control section 33 and the radio section 28 and is converted into a radio signal by the radio section 28 and then transmitted from the antenna 23.
The foldable portable telephone set 20 described above with reference to FIGS. 7 to 9 adopts a whip antenna for the antenna 23. A helical antenna 23a is provided at an upper end of the whip antenna, and in a state wherein the whip antenna is accommodated in the inside of the upper body 21a, the helical antenna operates as the antenna 23.
Generally, the antenna characteristic of a helical antenna is influenced much by the length of the housing (body) of a portable telephone set. Accordingly, in the foldable portable telephone set 20 whose housing length is much different depending upon whether it is folded or not folded, an optimum antenna matching circuit suitable for the housing length cannot be selected.